The present invention relates to a vehicle that may be braked with a motor as well as with a mechanical brake utilizing frictional force, and also to a method of controlling such a vehicle. More specifically the present invention pertains to a vehicle that is braked with a motor to attain an arbitrarily adjustable speed reduction rate, as well as to a controlling method to actualize such braking.
A hybrid vehicle with both an engine and a motor as the power source has been proposed as one form of transportation. For example, a hybrid vehicle disclosed in JAPANESE PATENT LAID-OPEN GAZETTE No. 9-37407 additionally has a motor placed in series between an engine and a transmission in a power system of an ordinary vehicle where an output shaft of the engine is connected with a drive shaft via the transmission. This arrangement enables the hybrid vehicle to be driven by means of both the engine and the motor as the power source. The engine generally has poor fuel consumption at a time of starting the vehicle. In order to avoid driving having poor fuel consumption, the hybrid vehicle makes a start by utilizing the power of the motor. After the speed of the vehicle reaches a predetermined level, the hybrid vehicle starts its engine and is subsequently driven by utilizing the power of the engine. The hybrid vehicle accordingly improves the fuel consumption at the time of starting. The hybrid vehicle causes the motor to regenerate the rotations of the drive shaft as electric power, which is used for braking (hereinafter such braking is referred to as the regenerative braking). The hybrid vehicle carries out the regenerative braking and thereby enables the kinetic energy to be used without significant waste. These characteristics desirably improve the fuel consumption of the hybrid vehicle.
There are two different types of braking in the vehicle. One braking process presses a brake pad against the drive shaft in response to actuation of a brake pedal, so as to apply frictional force to the axle (hereinafter referred to as the wheel braking). The other braking process causes the power source to apply a load to the drive shaft, like engine brake (hereinafter referred to as the power source braking). The hybrid vehicle utilizes, as the power source braking, engine brake based on a pumping loss of the engine and regenerative braking due to a regenerative load of the motor. The power source braking does not require the driver to change the foot position from the accelerator pedal to the brake pedal for the purpose of braking. In order to enhance the effectiveness of the power source braking, it is desirable to arbitrarily set a speed reduction rate required by the driver.
The engine brake results in a substantially fixed speed reduction rate according to the engine speed, unless the open and close timings of an intake valve and an exhaust valve are changed. In order to attain a desired speed reduction rate by engine brake, the driver is required to operate a gearshift level to vary the gear ratio of the transmission and thereby change the ratio of the torque of the power source to the torque output to the drive shaft. The advantage of the regenerative braking of the motor is, on the other hand, relatively easy control of the regenerative load, which leads to relatively easy control of the speed reduction rate. From this point of view, the hybrid vehicle disclosed in JAPANESE PATENT LAID-OPEN GAZETTE No. 9-37407 controls the regenerative speed reduction rate of the motor, in order to attain the desired speed reduction rate set by the driver.
The prior art hybrid vehicle, however, requires specific operations to change the setting of the speed reduction rate. The power source braking does not readily attain the driver""s desired speed reduction rate and is thus not utilized effectively enough. The required speed reduction rate frequently varies according to the driving state of the vehicle. In the prior art hybrid vehicle, the change of the actual speed reduction rate does not sufficiently follow the variation in required speed reduction rate. Namely the prior art hybrid vehicle has difficulties in subtle adjustment of the speed reduction rate.
In the prior art hybrid vehicle, the speed reduction rate can be set only in a changeable range of the regenerative load of the motor. In some cases, the hybrid vehicle can not sufficiently attain the speed reduction rate required by the driver. The insufficient speed reduction rate occurs especially in the course of high-speed driving of the vehicle.
Utilizing the wheel braking to compensate for the insufficiency of the speed reduction rate damages the advantage of the power source braking that effects the braking without any change of the foot position. The wheel braking causes the kinetic energy of the vehicle to be consumed in the form of thermal energy and accordingly damages the advantage of the hybrid vehicle that is the effective use of energy.
In the prior art hybrid vehicle, a large speed reduction rate may be attained by operating the gearshift lever to change the gear ratio of the transmission. In this case, however, the speed reduction rate drastically varies with the operation of the gearshift lever, which results in a poor ride.
The problems discussed above arise not only in the hybrid vehicle with both the engine and the motor as the power source but in any vehicle that is braked with the torque of the motor. The problems are also found in vehicles with a motor that is not used as the power source during a drive but is mounted for the purpose of regenerative braking.
One object of the present invention is thus to provide a vehicle that readily regulates the speed reduction rate in the process of braking with the torque of a motor by a compatible operation, which does not make the driver feel uneasy, as well as a method of controlling such a vehicle. Another object of the present invention is to provide a vehicle that enables smooth regulation of the speed reduction rate in a wide possible range of setting in response to a driver""s instruction, and a controlling method to attain such braking.
The present invention adopts the following arrangement, in order to attain at least part of the above and the other related objects.
The present invention is thus directed to a vehicle that is driven while regulating power output from a power source to a drive shaft through an operation of an accelerator unit. The vehicle includes: a motor that is capable of applying a braking force to the drive shaft; a detection unit that measures an operating amount of the accelerator unit; a target speed reduction rate setting unit that, when the observed operating amount of the accelerator unit is not greater than a predetermined level, sets a target speed reduction rate of the vehicle corresponding to the observed operating amount, based on a preset relationship between operating amount and speed reduction rate; a motor driving state specification unit that specifies a target driving state of the motor to apply a required braking force to the drive shaft, in order to attain the setting of the target speed reduction rate; and a control unit that controls the motor to be driven in the target driving state, so as to brake the vehicle.
The target driving state of the motor is specified with a diversity of driving state-related parameters, for example, a target torque, a quantity of electric power regenerated by the motor, and a value of electric current flowing through the motor.
In the vehicle of the present invention, the target speed reduction rate is set corresponding to the observed operating amount of the accelerator unit. The vehicle is under braking control with the setting of the target speed reduction rate. The accelerator unit is manipulated to specify a required magnitude of the power to be output from the power source. The accelerator unit generally has a margin of manipulation called play. When the operating amount of the accelerator unit is sufficiently small to be within a range of the play, the accelerator unit does not function to specify the magnitude of the output power. The vehicle of the present invention enables the driver to set the target speed reduction rate according to the operating amount of the accelerator unit in this range of the play. This arrangement thus allows the driver to readily adjust the speed reduction rate without feeling any incompatibility during a drive. The accelerator unit is a manipulation unit that is frequently operated during the drive. The vehicle of the present invention thus ensures adjustment of the target speed reduction rate, which well follows a variation in required speed reduction rate with a change of the driving state of the vehicle, while advantageously allowing minute adjustment of the target speed reduction rate.
The principle of the present invention is not restricted to the application of setting the target speed reduction rate in response to the operating amount of the accelerator unit in the range of the play. The predetermined level of the operating amount may be set to a value exceeding the range of the play. In the vehicle of the present invention, the accelerator unit that is manipulated to specify a variation in required power is also used to set the target speed reduction rate. For the purpose of making the conflicting instructions compatible, the effected instruction is changed, according to the range of the operation of the accelerator unit, between the specification of the required magnitude of the output power and the setting of the target speed reduction rate. This arrangement ensures the effects of the present invention discussed above. The predetermined level of the operating amount is thus not restricted to the range of the play, but may be set in any range suitable for the specification of the required power and the setting of the target speed reduction rate.
The following concretely explains the advantages of the arrangement that allows the target speed reduction rate to be set in response to the operation of the accelerator unit. The accelerator unit is typically constructed as an accelerator pedal. The braking control with the torque of the motor is effected in response to reduction of the depression of the accelerator pedal. When the reduced depression does not give a desired speed reduction rate corresponding to a driver""s requirement, the driver steps on a brake pedal to effect wheel braking and thereby raise the speed reduction rate. This requires a change of the foot position from the accelerator pedal to the brake pedal. Acceleration after the speed reduction requires another change of the foot position from the brake pedal to the accelerator pedal. Such frequent change of the foot position worsens the operatability of the vehicle.
In the vehicle of the present invention, however, the desired speed reduction rate corresponding to the driver""s requirement is achieved according to the reduced depression of the accelerator pedal as discussed above. The driver can thus brake the vehicle and re-accelerate the vehicle immediately after the speed reduction without any change of the foot position between the accelerator pedal and the brake pedal. Varying the step-on amount of the accelerator pedal allows minute adjustment of the speed reduction rate. The arrangement of the present invention thus significantly improves the operatability of the vehicle. The above discussion regards the accelerator unit constructed as the accelerator pedal. The advantages discussed above are, however, not restricted to such construction as the pedal.
Engine brake applies a practically unequivocally fixed speed reduction rate corresponding to the vehicle speed. A special mechanism is required to vary the speed reduction rate by the power source braking; for example, a mechanism of changing the on-off timings of an intake valve and an exhaust valve of the engine. The speed reduction rate by the motor, on the other hand, can be regulated rather easily and has a higher response. The vehicle of the present invention attains the desired speed reduction rate corresponding to the driver""s requirement, based on such advantages of the braking control by the motor.
The vehicle of the present invention has a further advantage on the energy efficiency. The wheel braking typically converts the kinetic energy of the vehicle into thermal energy through friction of the drive shaft against a brake pad and discharges the thermal energy to outside, so as to effect the braking control. This is not preferred from the viewpoint of the energy efficiency. The regenerative braking control by the motor, on the other hand, regenerates the kinetic energy of the vehicle in the form of electric power and thus allows the energy to be effectively used for a subsequent drive. The vehicle of the present invention allows a wide range of regenerative braking control by the motor, thus desirably enhancing the energy efficiency of the vehicle.
In the specification hereof, the speed reduction rate represents a parameter related to speed reduction of the vehicle; for example, a deceleration, that is, a decrease in vehicle speed per unit time, or a braking force.
In the specification hereof, the term xe2x80x98vehiclexe2x80x99 represents vehicles of diverse types. The first type includes vehicles using only a motor as the power source, that is, pure electric vehicles. The second type includes hybrid vehicles using both an engine and a motor as the power source. The hybrid vehicles are classified into two groups: parallel hybrid vehicles where the output power of the engine is directly transmittable to the drive shaft; and series hybrid vehicles where the output power of the engine is not directly transmitted to the drive shaft but is used only for power generation. The principle of the present invention is applicable to both the hybrid vehicles. The present invention may also be applied to vehicles with three or more prime movers including a motor as the power source. The third type includes vehicles using an engine as the power source for driving but having a motor for regenerative braking control.
The vehicle of the present invention may be provided with another braking force source to apply the braking torque, in addition to the motor. In the structure that has only the motor as the braking force source, the motor driving state specification unit specifies the target torque of the motor to effect all the desired speed reduction rate. The target torque generally takes a negative value, and the motor carries out regenerative operation. In the structure that has a plurality of different braking force sources including the motor, on the other hand, the motor driving state specification unit specifies the target torque of the motor by taking into account the speed reduction rate by the separate braking force source other than the motor. In the latter structure, the speed reduction rate by the other braking force source may be treated as a preset value. The torque of the motor may alternatively be subject to feedback control, in order to make the total speed reduction rate reach a predetermined level.
In the vehicle of the present invention, a diversity of settings may be applied for the predetermined relationship referred to by the target speed reduction rate setting unit.
Especially preferable is that the predetermined relationship referred to by the target speed reduction rate setting unit increases the speed reduction rate with a decrease in operating amount. One example of such setting is a relationship of decreasing the speed reduction rate inversely proportional to the operating amount.
The accelerator unit is generally designed to increase the power output from the power source in response to the greater operating amount. In other words, the accelerator unit is generally constructed to decrease the required power and lower the acceleration of the vehicle in response to the smaller operating amount. The arrangement of increasing the speed reduction rate with a decrease in operating amount of the accelerator unit well agrees with the driver""s feeling. The vehicle with the above setting for the predetermined relationship thus enables the driver to adjust the target speed reduction rate without feeling any incompatibility, thus ensuring the excellent operatability.
It is also preferable that the predetermined relationship referred to by the target speed reduction rate setting unit gives a significantly greater speed reduction rate in a specific state practically corresponding to the operating amount of zero than speed reduction rates in residual states.
Such setting for the predetermined relationship attains the speed reduction rate better suited to the driver""s feeling. The driver sets the operating amount of the accelerator unit equal to zero, that is, sets the accelerator unit in OFF position, when requiring rather abrupt braking control. The braking control at the significantly greater speed reduction rate in the OFF state of the accelerator unit than the speed reduction rates in the other states well follows the driver""s requirement. The vehicle of the above arrangement thus ensures the effective braking control by the motor.
A setting of continuously varying the speed reduction rate according to the operating amount may also be applicable to the vehicle of the present invention, where a reference speed reduction rate is set in the OFF state of the accelerator unit. In this case, however, the variation in speed reduction rate per unit operating amount of the accelerator unit, that is, the rate of change of the speed reduction rate, is relatively steep to ensure a sufficient level of the reference speed reduction rate. The large rate of change of the speed reduction rate makes it difficult to minutely adjust the speed reduction rate. The above setting for the predetermined relationship ensures the sufficient level of the reference speed reduction rate in the OFF state of the accelerator unit, while specifying the relationship between the speed reduction rate and the operating amount at the specific rate of change that enables minute adjustment of the speed reduction rate in the other states.
In the above setting of the predetermined relationship, the specific state practically corresponding to the operating amount of zero is defined by taking into account the resolving power of a sensor for measuring the operating amount of the accelerator unit. The specific state is not restricted to the state of setting the operating amount strictly equal to zero, but may include a range in which the operating amount is determined to be practically zero by taking into account the resolving power of the sensor.
When the vehicle of the present invention has a braking mechanism that utilizes mechanical frictional force, that is, a mechanism of wheel braking, the relationship between the operating amount of the accelerator unit and the speed reduction rate may be changed according to the on-off state of the wheel braking. One possible application sets the speed reduction rate by the motor in the ON state of the wheel braking to be greater than the speed reduction rate in the OFF state of the wheel braking. The driver generally requires a greater speed reduction rate in the ON state of the wheel braking. Such setting thus attains the braking control well suited to the driver""s feeling.
The relationship between the operating amount of the accelerator unit and the speed reduction rate may be specified comprehensively, based on a diversity of parameters.
For example, when the vehicle is provided with a vehicle speed detection unit that measures vehicle speed of the vehicle, the target speed reduction rate setting unit may set the target speed reduction rate, based on the observed operating amount and the observed vehicle speed.
In accordance with one preferable embodiment of the present invention, the vehicle further includes: a transmission that is capable of selecting one among a plurality of different gear ratios in the course of applying a braking force and is interposed between the motor and the drive shaft; a selection unit that selects a target gear ratio to attain the target speed reduction rate with a torque of the motor; and a change speed control unit that controls the transmission to effect the target gear ratio.
The vehicle of this construction attains an adequate gear ratio corresponding to the speed reduction rate specified by the driver and the magnitude of the torque of the motor by means of the selection unit. Controlling the driving state of the motor at the adequate gear ratio gives the speed reduction rate specified by the driver. The vehicle of this arrangement totally controls both the transmission and the motor, thus ensuring a wide range of braking control well following a driver""s requirement.
In the vehicle of this arrangement, the relationship between the operating amount of the accelerator unit and the target speed reduction rate may be changed according to the gear ratio currently used for driving.
In the vehicle with the transmission, it is preferable that the predetermined relationship referred to by the target speed reduction rate setting unit is specified to cause a variable range of the speed reduction rate corresponding to the operating amount to be allowed in a fixed gear ratio of the transmission.
Such setting for the predetermined relationship keeps the gear ratio of the transmission at a fixed value even when the speed reduction rate is adjusted by varying the operating amount of the accelerator unit. Namely the variation in speed reduction rate is implemented by controlling the motor. This arrangement enables adjustment of the speed reduction rate without changing the gear ratio, thus ensuring smooth driving.
The principle of the present invention is applicable to a diversity of vehicles having the power source of various structures.
It is especially preferable to apply the technique of the present invention to a vehicle with both the motor and an engine as the power source.
The present invention is desirably applied to a hybrid vehicle with both the motor and the engine mounted thereon, where the motor is used as the power source and the output power of the engine is transmittable to the axle. As mentioned previously, it is rather difficult to adjust the speed reduction rate by the engine brake in the vehicle with only the engine as the power source. In the hybrid vehicle with both the engine and the motor as the power source, on the other hand, the total speed reduction rate of the vehicle is adjusted rather easily by regulating the braking torque of the motor. Application of the present invention for the hybrid vehicle with the engine as the main power source significantly enhances the effectiveness of the power source braking.
The hybrid vehicle is preferably provided with the transmission discussed above. The hybrid vehicle of the above configuration generally has the motor as the auxiliary power source second to the engine. The motor is used, for example, at the time of starting the vehicle or during a low-speed drive, and is also utilized to supplement an insufficient of torque by the engine. The parallel hybrid vehicle typically has a small-sized motor of a relatively low output rating for this purpose. The motor alone does not have a sufficient ability of regenerative braking control required by the driver. The combined use of the transmission enables a wide range of braking control and ensures the especially effective application of the present invention.
In accordance with another preferable embodiment of the present invention, the vehicle further includes: a manipulation unit that is independent of the accelerator unit and enables a driver of the vehicle to specify a desired speed reduction rate in the course of braking control with the motor; and a change unit that changes a setting range of the target speed reduction rate of the vehicle according to the operating amount of the accelerator unit, in response to an operation of the manipulation unit.
In the vehicle of this embodiment, the driver operates the manipulation unit to change the setting range of the target speed reduction rate. The target speed reduction rate may be adjusted minutely according to the operating amount of the accelerator unit. This arrangement enables the driver to utilize the braking control by the motor in a wider range, thus significantly enhancing the operatability of the vehicle.
In the case of a requirement of a relatively large speed reduction rate, for example, in the case of preference of a crisp run over the good ride, the driver operates the manipulation unit to change the setting range of the target speed reduction rate to a higher side. Manipulation of the accelerator unit after such a change allows minute adjustment of the speed reduction rate about a relatively large reference speed reduction rate. When the road surface has a relatively low friction coefficient, for example, in the case of snow-covered road, the driver operates the manipulation unit to change the setting range of the target speed reduction rate to a lower side, in order to prevent the occurrence of a slip. Manipulation of the accelerator unit after such a change allows minute adjustment of the speed reduction rate about a relatively small reference speed reduction rate. The whole range of the speed reduction rate required by the driver may be set without any operation of the manipulation unit but through only the operation of the accelerator unit. The combination of the setting of the speed reduction rate through the operation of the manipulation unit with the adjustment through the operation of the accelerator unit advantageously facilitates the minute adjustment of the speed reduction rate.
In the vehicle having the independent manipulation unit for the setting of the speed reduction rate from the accelerator unit, the setting range of the target speed reduction rate may be changed in response to the operation of the manipulation unit, and the target speed reduction rate is adjusted according to the operating amount of the accelerator unit. The same effects may be achieved by different applications. One applicable method sets the target speed reduction rate in response to the operation of the manipulation unit and then corrects the target speed reduction rate according to the operating amount of the accelerator unit. Another applicable method sets the target speed reduction rate in response to the operation of the manipulation unit and then corrects the target torque of the motor according to the operating amount of the accelerator unit. These methods are only modifications of the processing and are practically equivalent to the above arrangement.
A diversity of structures may be applied for the manipulation unit.
For example, the manipulation unit may include a first switch that shifts the setting range in a stepwise manner in a direction of increasing the speed reduction rate, and a second switch that shifts the setting range in a stepwise manner in a direction of decreasing the speed reduction rate. The first switch and the second switch may be mounted on a steering wheel of the vehicle. This arrangement advantageously ensures the high operatability.
The manipulation unit may have a mechanism that allows the driver to specify the desired speed reduction rate by sliding a lever along a preformed slide groove. Especially the mechanism of continuously varying the setting of the speed reduction rate by a slide of the lever desirably heightens the degree of freedom in setting of the speed reduction rate.
In the case where the vehicle further includes: a transmission that is capable of selecting one among a plurality of different gear ratios for power output from the power source; and a gearshift lever that is operated to input a selected gearshift position, which represents a selectable range of the gear ratio during a drive of the vehicle, it is preferable that the manipulation unit and the gearshift lever have a common mechanism. This arrangement does not require a separate manipulation unit and desirably gives the manipulation unit of extremely high operatability.
In another preferable example, the manipulation unit has a slide groove, along which the gearshift lever is slid during a drive of the vehicle, and another slide groove, along which the gearshift lever is slid to specify the desired speed reduction rate, where these slide grooves are provided in series. This enhances the operatability in the process of specifying the speed reduction rate.
It is preferable that the vehicle with the manipulation unit for changing the setting range of the speed reduction rate has an information unit that gives the driver information regarding a setting state of the speed reduction rate. The information unit allows the driver to readily recognize the setting range of the speed reduction rate in response to the driver""s own operation. The information unit may be constructed in the form of a display unit that displays the setting range of the deceleration or in the form of an acoustic unit that informs the driver of the setting state of the speed reduction rate by acoustic means. The information given here may be any suitable piece of information; for example, information representing the setting range of the speed reduction rate or information representing a deviation from the reference speed reduction rate.
In accordance with still another preferable embodiment of the present invention, the vehicle further includes: a torque converter having a mechanism that converts the output power to another combination of torque and revolving speed by utilizing a slip between two rotating members and transmits the converted power, and a lock mechanism that locks up relative rotations of the two rotating members to allow direct transmission of the output power, the mechanism and the lock mechanism being provided on a pathway for transmitting the braking force of the motor to the drive shaft; and a lock mechanism regulation unit that, when the observed operating amount of the accelerator unit is not greater than a predetermined value, regulates the lock mechanism to fall into a specific state for restricting the slip between the two rotating members of the torque converter.
Regulating the lock mechanism during the braking control restricts the relative slip between the two rotating members of the torque converter. The braking force of the motor is thus transmitted to the drive shaft with little loss.
The torque converter may have a known mechanism utilizing a fluid.
In the application of regulating the lock mechanism, the specific state may lock up the relative rotations of the two rotating members. The predetermined value may be set in a range smaller than a specific operating amount to start braking control. This arrangement ensures the braking control well suited to the driver""s feeling. The present invention is, however, not restricted to such settings, but may be applicable to a diversity of other arrangements.
The vehicle with the torque converter may further include: a transmission that is capable of selecting one among a plurality of different gear ratios to transmit the braking force of the motor to the drive shaft; a gearshift position input unit that is manipulated to specify a selectable range of gear ratio of the transmission; and a mechanical braking mechanism that is operated to apply the braking force to the drive shaft by mechanical frictional force. In this application, the control unit regulates the gear ratio to allow selection of a greater gear ratio exceeding the selectable range specified by the gearshift position input unit, when the mechanical braking mechanism is operated.
Combination of the regulation of the gear ratio with the control of the motor attains the target speed reduction rate. In this embodiment, regulation of the gear ratio may exceed the restriction set by the gearshift position. This arrangement enhances the achievement of the target speed reduction rate. The control unit may be constructed as a unit of simply increasing the gear ratio by one step, irrespective of the specification of the gearshift position input unit, while the mechanical braking mechanism is being operated.
Other than the vehicles having any of the arrangements discussed above, the present invention may be actualized by a method of controlling such vehicles.